After extensive research and development, practical implantable cardioverter/defibrillators are now available for the medical community which can be used to effectively control tachyarrhythmia such as ventricular and/or atrial tachycardia as well as fibrillation. Preferably these devices are associated with, and in fact, incorporate a stimulation device that also provides bradycardia support.
Typically, an implantable cardioverter/defibrillator (hereinafter referred to simply as "defibrillator") monitors the patient's cardiac function for arrhythmia. Once arrhythmia is detected, the defibrillator analyzes and classifies the condition of the heart. For example, based on the rate of the ventricular beats and other factors, the patient's condition may be identified as low-rate tachycardia, high-rate tachycardia or fibrillation. The defibrillator then provides therapy specifically selected for the identified condition. The present invention pertains to therapy applied in response to tachycardia, and more particularly to therapy involving application of pacing pulses, and commonly referred to as antitachycardia pacing or ATP.
ATP consists of a three-step process used to revert the heart to normal sinus. First, ventricular tachycardia (VT) is detected and its interval (the VT interval) is determined. Second, a coupling interval is determined. Third, a delay is initiated corresponding to the coupling interval. At the end of the delay cardioversion pacing pulses are applied to the ventricle. The interval, number and frequency of the cardioversion pacing pulses are selected to capture the ventricle.
The selection of the coupling interval of the cardioversion pulses with respect to the intrinsic beats is very important. This interval is usually expressed or defined by a percentage relating the intrinsic ventricular interval and the coupling interval. Prior to the present invention this coupling percentage was defined as a parameter having one of two values. A relatively low coupling percentage was used for tachyarrhythmia of relatively low frequency. At a higher frequency (for example, above a preselected threshold), a higher coupling percentage was used. The result was coupling interval which was relatively longer at higher VT rates and provides proportionately a more aggressive antitachycardia therapy at lower VT rates.
A major problem with the approach described above is that the coupling percentage is initially set to at most one or two values, typically selected on implantation by the physician. This approach assumes either that the coupling percentage is not an important parameter and hence one or two values are enough, or that the same value(s) can be used for patients at all times. However, studies have shown that there is no universal optimal percentage but, to the contrary, the optimal percentage may not only vary from patient to patient but also from one VT episode to another even for the same patient.
The present invention addresses this problem and provides a method and apparatus wherein the coupling percentage and/or coupling interval is adjusted automatically on the fly thereby insuring that an optimal coupling percentage is reached rapidly and aggressively before the patient's condition advances to a potentially dangerous vertical fibrillation.